Because MOS field effect transistors (hereinafter referred to as “MOSFETs”) have higher input impedance than bipolar transistors, their power gains are high and their gate driving circuits are very simple. Generally, when devices are turned off, minority carrier storage or minority carrier recombination causes time delay. However, because the MOSFET is a unipolar device, it does not experience any such time delay. Thus, applications (e.g., switching mode power supplies, lamp ballast and motor driving circuits), for MOSFETs are expanding. MOSFETs usually adopt the DMOSFET(double diffused MOSFET) structure embodied by planar diffusion technology. A typical LDMOS transistor is described in Sel Cloak, U.S. Pat. No. 4,300,150. Also, an LDMOS transistor integrated with a CMOS transistor and a bipolar transistor, was described on pages 322–327 of the “ISPSD 1992” in a paper entitled “A 1200 BiCMOS Technology and Its Application”, by Vladimir Rumennik and on pages 343–348 of the “ISPSD 1994” in a paper entitled “Recent Advances in Power Integrated Circuits with High Level Integration,” by Stephen P, Robb.
It is important for DMOS transistors to be employed with power devices which can handle high voltage. Such power devices have good current handling capacity per unit area or a good ON-resistance per unit area. Because the voltage ratio is fixed, the ON-resistance per unit area can be reduced by decreasing a cell area of the MOS device.
In the field of power transistors, a cell pitch of a device is determined by the combined width of a polysilicon region and a contact region, which form a gate electrode and a source electrode, respectively. Reducing a P-type well junction depth is a well-known technique for diminishing the width of a polysilicon region in a DMOS power transistor. However, the breakdown voltage restricts the junction depth.
A conventional LDMOS device is well suited to VLSI processes due to its simple structure. Nevertheless, these LDMOS devices have been regarded as less attractive than VDMOS (vertical DMOS) devices. Recently, RESURF (reduced surface field) LDMOS devices have been shown to have a good ON-resistance characteristic. However, their structure is very complex, they can only be applied for devices whose sources are earthed, and they are difficult to use in other applications.
Particularly, in the past, DMOS transistors were used as discontinuous power transistors or elements of monolithic integrated circuits. Because DMOS transistors are fabricated using a self-aligned manufacturing procedure, they basically comprise a semiconductor substrate.
To form a self-aligned channel region with a gate electrode, a channel body region is generally formed by implanting either p-type dopants or n-type dopants through apertures within a mask, which is made of materials for the gate electrode. A source region is formed by implanting conductive dopants opposite to what are used for the channel body region. The source region is then self-aligned to both the gate electrode and the channel body region. This is a reason why the DMOS transistor has a compact structure.
Referring to FIG. 1, an LDMOS transistor device 10 actually has two LDMOS transistors 10a and 10b. 
The transistor device 10a is formed on a SOI (silicon on insulator) substrate comprising a silicon substrate 11, a buffer oxide layer 12 and a semiconductor layer 14. In the illustrated example, the semiconductor layer 14 is formed over the silicon substrate 11. This prior art FET (field effect transistor) comprises a source region 16a and a drain region 18a. The N-type doped source region 16a is formed within a P-type doped well region 20. The well region 20 is often called a P-type body. The P-type body 20 may extend to the upper surface of the buffer oxide layer 12 or be only within the semiconductor layer 14.
The drain region 18a is in contact with one end of a field insulation region 23a. The field insulation region 23a includes a field oxide layer such as a thermally grown silicon oxide layer.
A gate electrode 26a is formed on the surface of the semiconductor layer. The gate electrode 26a extends from the upper part of the source region 16a to the upper part of the field insulation region 23a. It is made of polysilicon doped with impurities. The gate electrode 26a is isolated from the semiconductor layer 14 by a gate dielectric 28a. The gate dielectric 28a may comprise oxide, nitride or a combination thereof (e.g., a stacked NO or ONO layer)
Sidewall insulation regions (not shown) may be formed on the sidewalls of the gate electrode 26a. The sidewall insulation regions commonly comprise oxide such as silicon oxide or nitride such as silicon nitride.
A body region 30 doped at a high concentration exists within the P-type body 20. This body region 39 is in good contact with the P-type body 20. It is doped at a higher concentration than the P-type body 20.
A source contact plug 34 and a drain contact plug 32a exist within the transistor device 10a. These contact plugs (34 and 32a) are provided to electrically connect the source region 16a and the drain region 18a to other elements of the circuit. Referring to FIG. 1, the single contact plug 34 is used for the source regions, 16a and 16b, of the two transistors,10a and 10b. The prior art technology described above is further described in Ng et al., U.S. Pat. No. 5,369,045.
However, during a diffusion process for forming channels, the prior art requires a thermal treatment process at a high temperature, which affects devices badly.